WASHINGTON — The Obama administration geared up for the biggest foreign policy vote since the Iraq war by arguing Sunday that new physical evidence shows the Syrian government used sarin gas in a deadly August attack. With its credibility on the line, the United States must respond, the country’s top diplomat said.

Members of Congress, deadlocked on just about everything these days and still on summer break, expressed sharply divergent opinions about whether to give President Barack Obama the go-ahead he requested to retaliate with military force against the Assad regime, and what turning down the commander in chief could mean for America’s reputation.

Presenting Obama’s case for military action, Secretary of State John Kerry gave a series of interviews on Sunday news shows outlining the latest information the administration has received about the Aug. 21 attack in the Damascus suburbs that the U.S. says killed 1,429 civilians, including more than 400 children. He said samples collected by first responders added to the growing body of proof that Syria’s government launched a chemical weapons attack.

“Samples of hair and blood have been tested and they have reported positive for signatures of sarin,” Kerry said. “Each day that goes by, this case is even stronger. We know that the regime ordered this attack. We know they prepared for it. We know where the rockets came from. We know where they landed. We know the damage that was done afterwards.”

Sarin, which affects the nervous system and is toxic in liquid or gas form, can be delivered in missiles, bombs, rockets or artillery shells. The gas is outlawed under international rules of warfare. The reference to hair and blood samples were the first pieces of specific physiological evidence cited by any member of the administration, which previously spoke only about an unnamed nerve agent.

Kerry’s assertion coincided with the beginning of a forceful administration appeal for congressional support, now that Obama has declared he will await approval from the House and Senate before ordering any cruise missile strikes or other action.

On Capitol Hill, senior administration officials briefed lawmakers in private to explain why the U.S. is compelled to act against President Bashar Assad’s government. Obama, Vice President Joe Biden and White House Chief of Staff Denis McDonough also made calls to individual lawmakers. Further classified meetings were planned over the next three days.

Sen. John McCain of Arizona, a leading Senate hawk and the candidate Obama defeated for the presidency in 2008, said he’d discuss Syria with the president at the White House today.

Obama must convince skeptical Americans and their representatives in Congress of the need for more U.S. military action in the Muslim world after a decade of war in Iraq and Afghanistan.

He also is trying to assemble an international coalition, but finding it hard to land partners. They fear becoming involved in a conflict that has claimed more than 100,000 lives in the past 2½ years and dragged in terrorist groups on both sides of the battlefield.

Only France is firmly on board among the major military powers. Britain’s Parliament rejected the use of force in a vote last week.

The United Nations on Sunday asked the head of its chemical weapons inspection team to expedite the analysis of tests from samples it collected from Syria last week.

Assad’s government, which has denied allegations of chemical weapons use, reveled in Obama’s decision to defer any immediate action. Deputy Foreign Minister Faysal Mikdad claimed that the move reflected the lack of evidence of government culpability.

With Navy ships on standby in the eastern Mediterranean sea ready to launch missiles, Congress began a series of meetings that will take place over the next several days in preparation for a vote once lawmakers return from summer break, which is scheduled to end Sept. 9.

Dozens of members attended the two-hour classified briefing Sunday in the Capitol, though many emerged saying they needed to see more details of Obama’s plan and more facts about the alleged chemical weapons attack. Many feared giving Obama overly broad authority for military action.

On selling the strategy to Congress, Rep. Rep. Bennie G. Thompson, the senior Democrat on the House Homeland Security Committee, said, “They have a ways to go.”

“They also have work to do with respect to shoring up the facts of what happened,” Thompson said.

The Senate Foreign Relations Committee plans a meeting Tuesday, according to its chairman, Sen. Bob Menendez, D-N.J. The Senate Armed Service Committee will gather a day later, said Oklahoma Sen. Jim Inhofe, the top Republican on the panel.

Kerry confidently predicted that lawmakers would back limited military strikes.

“The stakes are just really too high here,” he said.

Kerry was asked repeatedly in the broadcast interviews what Obama would do if Congress didn’t give its consent. He said he believed lawmakers would recognize the grave implications for letting a chemical weapons attack go unchecked and what that might mean for U.S. efforts to force North Korea to get rid of its nuclear weapons and prevent Iran from acquiring such capability.

“We are not going to lose this vote,” Kerry said. “The credibility of the United States is on the line.”

Obama is likely to find stronger support in the Democrat-controlled Senate than the GOP-dominated House, yet faces complicated battles in each. Some anti-war Democrats and many tea party-backed Republicans are opposed to any intervention at all, while hawks in both parties, such as McCain, feel the president must do far more to help Syria’s rebels oust Assad from power.

In an interview with an Israeli television network, McCain said Obama has “encouraged our enemies” by effectively punting his decision to Congress. He and fellow Republican Sen. Lindsey Graham of South Carolina have threatened to vote against Obama’s authorization if it is too limited.

On the other end of the spectrum, an unusual coalition of foreign policy isolationists, fiscal conservatives and anti-interventionists in both parties opposes even limited action for fear that might draw the United States into another costly and even bloody confrontation.

The White House request to Congress late Saturday speaks only of force to “deter, disrupt, prevent and degrade” the Assad regime’s ability to use chemical weapons.

“I think it’s a mistake to get involved in the Syrian civil war,” said Sen. Rand Paul, R-Ky.

Echoing that sentiment, Sen. Chris Murphy, D-Conn., questioned, “Does a U.S. attack make the situation better for the Syrian people or worse?”

Paul expected the Senate to “rubber-stamp” Obama’s plan, while he said it was “at least 50/50 whether the House will vote down involvement in the Syrian war.” Inhofe predicted defeat for the president.

Despite the intense gridlock in Congress over debt reduction, health care, immigration and other issues, some lawmakers were more optimistic about the chances of consensus when it came to a question of national security.

Rep. Peter King, R-N.Y., who criticized Obama for not proceeding immediately against Assad, said he’d vote “yes” and believed the president should be able to build a House majority over the next several days.

“At the end of the day, Congress will rise to the occasion,” added Rep. Mike Rogers, R-Mich. “This isn’t about Barack Obama versus the Congress. This isn’t about Republicans versus Democrats. This has a very important worldwide reach.”

Sen. Tim Kaine, D-Va., said Congress and the American people would support action once Obama finishes making his case. Sen. Saxby Chambliss, R-Ga., said if Obama doesn’t do that, he won’t get his authorization.

“He’s got to come out and really be in-depth with respect to the intelligence that we know is out there,” said Chambliss, the top Republican on the Senate Intelligence Committee. “He’s got to be in-depth with respect to what type of military action is going to be taken and what is our current strategy.”

At the Capitol, Sen. Jeff Sessions, R-Ala., said Obama’s proposed resolution needed tightening. “I don’t think Congress is going to accept it as it is,” he said.

In his TV interviews, Kerry reiterated Obama’s oft-repeated promise not to send any American troops into Syrian territory.

Polls show significant opposition among Americans to involvement, and several lawmakers have cited the faulty intelligence about weapons of mass destruction that led up to President George W. Bush’s 2003 Iraq invasion as justification of the need for lengthy debate before U.S. military action.

Kerry, who voted to authorize Bush’s 2003 Iraq invasion but then opposed it in his unsuccessful presidential bid a year later, rejected any comparisons to America’s recent wars.

“This is not Iraq. This is not Afghanistan. There is nothing similar in what the president is contemplating,” Kerry said. “There are others who are willing to fight, others who are engaged. And the issue here is not whether we will go and do it with them, it’s whether we will support them adequately in their efforts to do it.”

Kerry appeared on CBS, NBC’s “Meet the Press,” CNN’s “State of the Union,” “Fox News Sunday” and ABC’s “This Week.” Paul was on NBC, Rogers and Murphy were on CNN, King and Inhofe were on Fox, and Chambliss and Kaine were on CBS.